1. Technical Field
Embodiments of the present disclosure are related generally to fluid handling in hydraulic power transmission applications, and in particular to hydraulic fluid manifolds.
2. Description of the Related Art
Hydraulic machines are in common use in a wide variety of industrial and commercial applications. Hydraulic machines transmit power by conducting pressurized fluid between low pressure and high pressure reservoirs. Fluid is typically conducted by means of hoses or pipes joined by one or more fluid conducting manifolds that facilitate their connection. It is also known in the art to provide valve means integrated within the body of such manifolds by which flow may be selectively routed in response to operating conditions.
Typically, the low pressure side of a hydraulic circuit will have one or more manifolds dedicated to handling low pressure flow. Auxiliary functions such as fluid filtration or cooling are also likely to reside on the low pressure side, adding to the number of connections that might be plumbed in to the manifold. These auxiliary functions may also require additional flow regulating means such as auxiliary pumps, check valves or proportional valves, adding to the complexity of the circuit.
Application of hydraulic machines and hydraulic circuits to hybrid vehicle powertrains is a relatively new area. These applications present special performance considerations. For fuel efficiency it is important to provide for efficient handling of regenerative braking, which can create very high fluid flow rates at short notice and in a reverse direction from the usual flow. For driver and passenger safety it is also important for the low pressure circuit to provide a reliable pressure relief function to allow pressurized fluid to safely retreat to the low pressure reservoir in case a component failure or other anomaly occurs, such as for example, a “blow-off” event in which the cylinder barrel of a hydraulic motor becomes unseated. In general, it is also important to minimize flow restrictions wherever fluid traverses the manifold at a high flow rate in order to maximize energy efficiency of the vehicle. Vehicular applications are also particularly sensitive to issues that affect weight, packaging, cost, and manufacturability. Ideally the low pressure manifold should integrate into its design as much of the above described functionality as possible, to reduce the number of individual components that must be separately mounted and installed on the vehicle. The hydraulic manifold art has yet to develop standard solutions for these needs. The design of a low pressure manifold for a vehicular application therefore remains a challenging and inventive task.
As suggested previously, one function of such a manifold would be to allocate flow to auxiliary functions such as cooling and filtration of the working fluid. A manifold body can easily provide for multiple fluid passages as well as convenient mounting and integration of valve means to regulate flow within the passages. One might therefore provide for connections to auxiliary paths on which the auxiliary means reside, and selectively induce fluid flow from a primary fluid path into the auxiliary paths by selectively constricting the main fluid path by means of an integrated proportional valve.
The manifold would also need to support rapid discharge to the low pressure side, as well as regenerative flows in a reverse direction. The presence of a proportional valve on the main path might restrict such flows if they were to occur when the proportional valves are closed or partially closed. For example, if a blow-off event were to suddenly occur, the filter and coolers may fail or rupture before the control system has time to detect and respond to the condition by fully opening the proportional valves. As another example, when a regenerative braking event occurs, a slow response might cause a significant portion of the recoverable energy to be lost by passing through the filter or cooler, or through partially closed proportional valves, rather than being more efficiently recovered through a less restricted path. However, a manifold may easily provide for the additional passages and valve means necessary to allow such flows to bypass the proportional valves. For example, regenerative braking flows could be supported independently of the main path by a parallel passage fitted with a flow checking means (such as for example a check valve) to allow only flows toward the high pressure side. Blow-off flows could be supported by another parallel passage with a spring-loaded check valve to allow only particularly large flows toward the low pressure side.
An even greater advantage could be realized if, rather than having separate flow checking means on parallel fluid paths, the function of each of these added paths and flow checking means could be performed by a single valve in the main path. This would call for a unique valve design to be integrated with the manifold.
One form of valve suitable for use in a manifold is the butterfly valve. Butterfly valves are well known in the art and typically include a flat valve member (such as a disc or similar shape) disposed in a fluid channel. The disc is fixed to a rotatable shaft (called a stem) having an axis intersecting the plane of the disc at a point near its center. When the plane of the disc is perpendicular to the fluid channel, the fluid channel is substantially closed to flow. When the plane of the disc is parallel to the fluid channel, the channel is maximally open to flow. The angular positions in between establish proportional control of fluid flow across the valve.
In some applications, a direct actuator such as a hand wheel or a servo motor is used to turn the stem and thereby rotate the disc to a specific angle. These valves typically have a disc that is divided by the axis of the stem into two equally sized wings.
In other applications, the disc is not directly actuated to an angle but instead is biased to a closed position and takes on a flow-induced opening angle in response to the action of fluid flow across the valve. These valves typically have what is commonly known in the art as an offset butterfly disc, in which the axis of the stem divides the disc into two unequal wings, one larger than the other, thereby making the larger wing follow the direction of fluid flow and thereby causing the valve member and stem to rotate in a first direction in response to fluid flow in a first direction, and in a second opposite direction in response to fluid flow in the opposite direction. If the pressure differential across the valve is sufficient to overcome the biasing force, the valve will crack open and fluid flow across the valve will exert a torque rotating the valve to an angle generally corresponding to the rate of flow, until the flow diminishes enough to allow the biasing force to close it again.
A flow-induced butterfly valve with biasing force may therefore act as a sort of relief valve or limited-range check valve that blocks flow only up to a predetermined flow-induced pressure differential. The biasing force is provided by a biasing means, typically including a spring or similar elastic or resilient component, and will vary with the angle of rotation along what could be called a biasing force profile. For example, a valve biased by an ordinary spring would be expected to present a biasing force profile that is a generally linear function of deflection, because spring force F=kx, where k represents the spring constant of a given spring, and x is the deflection.
Unfortunately, standard butterfly valves do not meet every need posed by a vehicular hydraulic manifold. First, most flow-induced butterfly valves are designed to allow flow in one direction and block flow entirely in the other. This would be acceptable in a manifold having parallel unidirectional flow passages but not one that seeks to combine, for example, both regenerative braking and blow-off flows in a single passage. Second, the biasing spring will tend to limit the degree to which fluid flow alone may open the valve. A butterfly disc biased by an ordinary torsion spring having a spring force sufficient to create a strong biasing force at the closed position may be practically incapable of being opened by fluid flow alone beyond an angle of 70 to 80 degrees, perhaps much less. This is largely because the torque exerted on a butterfly disc by fluid flow drops off dramatically as the disc becomes more parallel to the flow direction, until the flow can no longer overcome the biasing force necessary to further rotate the valve.
It would be preferable to provide a selectively controllable biasing means that would allow the biasing force to be selectively reduced so as to allow nearly full opening of the valve (approaching 90 degrees) in order to minimize flow obstruction at high flow rates, where the greatest impact on overall system efficiency is felt. It would also be preferable to allow bidirectional flow, providing a different bias in each direction so that two directions of fluid flow, such as that supporting a drive mode and blow off mode in one direction, and a regenerative braking mode in the opposite direction, can be accommodated in a single flow path, with specific levels of biasing appropriate to each. A butterfly valve having these features would make it particularly suitable for use in a vehicular hydraulic manifold.
Butterfly valves, valve actuators, and hydraulic manifolds are described in a large number of patents, including the following examples and many others. The bulk of prior art in the field of butterfly valves, similar types of valves, and their actuators focuses on direct actuation of the valve to a specific rotation angle (for example, U.S. Pat. No. 4,132,071, No. 4,261,546; No. 7,028,979), or on making the valve return to a closed or open position on failure of the actuator (for example, U.S. Pat. No. 4,132,071), or for faster closing of the valve (U.S. Pat. No. 5,671,903; No. 4,556,192) or for modifying the biasing force acting on the rotation of the disc (for example, U.S. Pat. No. 6,648,013 in which the force necessary to keep the valve open after initially cracking open is reduced, or U.S. Pat. No. 6,938,597 in which the force biasing an intake valve is controllably varied). To the knowledge of applicant there has been no example of a bi-directional butterfly valve with a different biasing force in opposite directions and having an actuator that controls a biasing force in one of the directions, nor have there been examples of the use of such a valve to advantageously regulate fluid flow in a hydraulic manifold.